﻿ON 
  THE 
  NORTHERN 
  SLOPES 
  OF 
  CADER 
  IDRI8. 
  427 
  

  

  chloric 
  acid, 
  a 
  residue 
  of 
  dusky 
  grey 
  or 
  colourless 
  matter 
  remains. 
  

   In 
  this 
  some 
  pumiceous 
  particles 
  may 
  be 
  made 
  out 
  ; 
  but 
  the 
  oolitic 
  

   grains 
  themselves 
  leave 
  a 
  siliceous 
  residue, 
  which 
  no 
  longer 
  affects 
  

   polarized 
  light, 
  although 
  the 
  concentric 
  structure 
  is 
  retained. 
  It 
  

   will 
  be 
  seen 
  how 
  the 
  discussion 
  of 
  the 
  Liassic 
  ironstone 
  of 
  Cleveland 
  

   by 
  Mr. 
  Sorby 
  * 
  and 
  of 
  the 
  Northamptonshire 
  ores 
  by 
  Prof. 
  Judd 
  f 
  

   applies 
  also 
  to 
  this 
  older 
  and 
  more 
  metamorphosed 
  deposit. 
  

  

  In 
  connexion 
  with 
  Prof. 
  Judd's 
  observations 
  on 
  the 
  presence 
  of 
  

   phosphoric 
  acid 
  in 
  the 
  greener 
  varieties 
  of 
  the 
  Northampton 
  ore 
  £, 
  

   we 
  may 
  remark 
  that 
  the 
  ironstone 
  of 
  Cader 
  Idris 
  gives 
  a 
  distinct 
  

   reaction 
  when 
  treated 
  with 
  a 
  nitric-acid 
  solution 
  of 
  ammonic 
  molyb- 
  

   date. 
  

  

  Thus 
  we 
  are 
  led 
  to 
  regard 
  the 
  oolitic 
  structure 
  in 
  this 
  ironstone 
  

   as 
  original. 
  The 
  carbonate 
  of 
  iron 
  which, 
  in 
  all 
  probability, 
  replaced 
  

   the 
  calcite 
  or 
  aragonite 
  of 
  the 
  grains 
  has 
  now 
  itself 
  been 
  broken 
  

   up, 
  leaving 
  the 
  cubes 
  of 
  magnetite 
  as 
  a 
  residue. 
  The 
  rock 
  still 
  

   effervesces 
  slightly 
  when 
  boiled 
  in 
  hydrochloric 
  acid 
  ; 
  but 
  the 
  ease 
  

   with 
  which 
  its 
  powder 
  is 
  attracted 
  by 
  the 
  magnet 
  shows 
  that 
  the 
  

   bulk 
  of 
  the 
  iron 
  is 
  now 
  in 
  the 
  oxidized 
  condition. 
  The 
  alteration 
  

   having 
  taken 
  place 
  as 
  a 
  deep-seated 
  process 
  and 
  not 
  in 
  contact 
  

   with 
  the 
  air, 
  neither 
  haematite 
  nor 
  limonite 
  has 
  been 
  developed. 
  A 
  

   specimen 
  of 
  the 
  pisolitic 
  ironstone 
  of 
  Pen-y-morfa 
  near 
  Tremadoc, 
  

   for 
  which 
  we 
  are 
  indebted 
  to 
  Mr. 
  G. 
  J. 
  Williams, 
  E.G.S., 
  is 
  still 
  more 
  

   strikingly 
  magnetic. 
  

  

  If 
  the 
  iron-ore 
  of 
  Cader 
  Idris 
  was 
  at 
  one 
  time 
  a 
  fairly 
  calcareous 
  

   band 
  among 
  the 
  more 
  prevalent 
  muds 
  and 
  shales, 
  it 
  owes 
  its 
  preser- 
  

   vation 
  to 
  the 
  pseudomorphic 
  action 
  that 
  has 
  gone 
  on. 
  The 
  very 
  

   marked 
  proportion 
  of 
  carbonate 
  of 
  lime 
  that 
  occurs 
  in 
  the 
  cavities 
  

   of 
  the 
  permeable 
  rocks 
  from 
  Llyn 
  Gwernen 
  to 
  the 
  summit 
  of 
  the 
  

   mountain 
  may 
  have 
  been 
  in 
  part 
  derived 
  from 
  similar 
  seams 
  or 
  shelly 
  

   patches, 
  the 
  absence 
  of 
  which 
  we 
  have 
  so 
  frequently 
  to 
  deplore 
  when 
  

   examining 
  Ordovician 
  or 
  Cambrian 
  strata. 
  It 
  is 
  unnecessary 
  to 
  

   dwell 
  upon 
  the 
  increased 
  effect 
  of 
  solvent 
  action 
  when 
  occurring 
  

   in 
  a 
  region 
  subjected 
  to 
  pressure 
  and 
  earth-movement. 
  

  

  On 
  the 
  east 
  flank 
  of 
  Tyrau 
  Mawr, 
  and 
  above 
  the 
  mass 
  of 
  the 
  

   main 
  eurite, 
  volcanic 
  beds 
  with 
  slaty 
  fragments 
  recur 
  at 
  a 
  height 
  

   of 
  eighteen 
  hundred 
  feet 
  (fig. 
  3). 
  At 
  this 
  point 
  the 
  junction 
  of 
  the 
  

   tuff 
  with 
  the 
  overlying 
  normal 
  slate 
  is 
  marked 
  by 
  a 
  thin 
  white 
  band 
  

   consisting 
  of 
  granules 
  of 
  quartz 
  and 
  decomposed 
  felspar 
  ; 
  it 
  would 
  

   seem 
  that 
  this 
  sand 
  is 
  a 
  washed 
  residue 
  from 
  the 
  uppermost 
  layers 
  

   of 
  the 
  tuff, 
  such 
  as 
  would 
  be 
  formed 
  by 
  the 
  action 
  of 
  waves 
  or 
  

   currents 
  prior 
  to 
  the 
  next 
  sedimentation. 
  

  

  Close 
  to 
  the 
  foot 
  of 
  the 
  Roman 
  road, 
  where 
  it 
  descends 
  in 
  bold 
  

   zigzags 
  from 
  Craig 
  Glas, 
  the 
  talus 
  is 
  strewn 
  with 
  blocks 
  of 
  coarse 
  

   felspathic 
  tuff, 
  which 
  include 
  wisps 
  of 
  shale 
  more 
  conspicuous 
  than 
  

   those 
  of 
  Penrhyn-gwyn. 
  The 
  dimensions 
  of 
  the 
  surface 
  here 
  figured 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxv. 
  (1879V 
  Proc. 
  p. 
  84. 
  

   t 
  Mem. 
  Geol. 
  Survey, 
  "Geology 
  of 
  Rutland," 
  pp. 
  117-138. 
  

   } 
  Loc. 
  cit, 
  p. 
  127. 
  

  

  